Myeloperoxidase Neuron Specific Enolase P-Selectin antigen Platelet Factor 4 antigen PMN Elastase Prekallikrein factor activity Prothrombin fragment 1.2 S100 Soluble endothelial protein C receptor Tissue Factor antigen, soluble Thrombin antithrombin complex antigen Thrombomodulin antigen Tissue Factor antigen Tissue Factor Pathway Inhibitor antigen Hemostasis is a complex process in which multiple components of the blood clotting system are activated in response to vessel injury to control bleeding. Hemostasis is composed of four major events: 1. Primary hemostasis 2. Secondary hemostasis 3. Fibrin clot formation and stabilization 4. Inhibition of coagulation 1. Primary hemostasis = vasoconstriction and platelet plug formation: ▲ The key component of primary hemostasis is the platelet. ▲ Primary hemostasis is triggered by injury to the vessel wall, exposing subendothelial collagen. ▲ Vasoconstriction occurs at the site of injury to reduce blood flow. ▲ Adhesion:von Willebrand factor adheres platelets to exposed subendothelial collagen via the platelet receptor glycoprotein Ib (GPIb). Platelets also adhere directly to collagen by other receptors. ▲ Aggregation:Platelets aggregate with each other with the help of fibrinogen that binds to activated glycoprotein IIb-IIIa (GPIIb / IIIa), forming a platelet plug. Platelet aggregates also provide the phospholipid surface necessary for coagulation factor activation. 2. Secondary hemostasis = activation of coagulation factors and generation of thrombin: Initiation of coagulation ▲ Tissue factor (TF) is released from injured tissue cells, endothelial cells and monocytes. ▲ TF and Factor VIIa form the TF / Factor VIIa complex. ▲ TF / Factor VIIa activates a small amount of Factor IX and X to generate a small amount of thrombin. ▲ Factor XII (and other “contact” factors) play a minor role in the activation of Factor XI. Amplification phase ▲ Thrombin activates Factor V to Va, Factor VIII to VIIIa and activates more platelets. ▲ Thrombin also activates FXI to FXIa. Propagation phase ▲ Additional Factor Xa is produced when TF / Factor VIIa complex activates Factor IX. The resultant Factor IXa along with Factor VIIIa forms the tenase complex which then converts more Factor X to Xa. ▲ Factor Xa and Va along with calcium and a phospholipid (PL) surface (activated platelets) form the prothrombinase complex which converts prothrombin (Factor II) to large amounts of thrombin (Factor IIa). 3. Fibrin clot formation and stabilization: ▲ Thrombin converts fibrinogen to fibrin monomers which polymerize to form a soluble clot. Thrombin then activates Factor XIII which cross-links the fibrin monomers and stabilizes the clot. 5 GPlb / IX VWF Injury site collagen exposed Platelet Collagen Endothelium GPllb / llla Dense granules Alpha granules Activated Platelet GPllb / IIIa GPlb / IX VWF Fibrinogen Collagen 6 1. Coagulation & Clot Breakdown 1. THE BASICS OF COAGULATION AND CLOT BREAKDOWN 4. Inhibition of coagulation = inhibition of thrombin generation and fibrin clot breakdown (fibrinolysis) Inhibition of thrombin generation ▲ At the same time that a clot is being formed, the clotting process also starts to shut itself off to limit the extent of the thrombus formed. ▲ Thrombin binds to the membrane receptor thrombomodulin and activates Protein C to Activated Protein C (APC). ▲ APC combines with its cofactor Protein S which then inhibits Factors Va and VIIIa, slowing down the coagulation process. ▲ Thrombin bound to thrombomodulin becomes inactive and can no longer activate procoagulant factors or platelets. ▲ The endogenous anticoagulant, antithrombin inhibits the activity of thrombin as well as several of the other activated factors, primarily Factor Xa. Fibrinolysis ▲ Tissue plasminogen activator (t-PA) converts plasminogen to plasmin which breaks down cross-linked fibrin to several fibrin degradation products, the smallest of which is D-dimer. ▲ Thrombin activatable fibrinolysis inhibitor (TAFI) prevents the formation of plasmin. ▲ Anti-plasmin and plasminogen activator inhibitor-1 (PAI-1) inhibit plasmin and t-PA respectively. 7 PAI - 1 Fibrin Clot Plasmin Plasminogen tPA Antiplasmin TAFI Fibrin(ogen) Degradation Products D-dimers INITIATION